Double dissociation between the involvement of the bed nucleus of the stria terminalis and the central nucleus of the amygdala in startle increases produced by conditioned versus unconditioned fear

The amplitude of the acoustic startle response is reliably enhanced when elicited in the presence of bright light (light-enhanced startle) or in the presence of cues previously paired with shock (fear-potentiated startle). Light-enhanced startle appears to reflect an unconditioned response to an anxiogenic stimulus, whereas fear-potentiated startle reflects a conditioned response to a fear-eliciting stimulus. We examine the involvement of the basolateral nucleus of the amygdala, the central nucleus of the amygdala, and the bed nucleus of the stria terminalis in both phenomena. Immediately before light-enhanced or fear-potentiated startle testing, rats received intracranial infusions of the AMPA receptor antagonist 2, 3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)-quinoxaline (3 microg) or PBS. Infusions into the central nucleus of the amygdala blocked fear-potentiated but not light-enhanced startle, and infusions into the bed nucleus of the stria terminalis blocked light-enhanced but not fear-potentiated startle. Infusions into the basolateral amygdala disrupted both phenomena. These findings indicate that the neuroanatomical substrates of fear-potentiated and light-enhanced startle, and perhaps more generally of conditioned and unconditioned fear, may be anatomically dissociated.     

Evidence of contextual fear after lesions of the hippocampus: a disruption of freezing but not fear-potentiated startle

The roles of the dorsal hippocampus and the central nucleus of the amygdala in the expression of contextual fear were assessed using two measures of conditioned fear: freezing and fear-potentiated startle. A discriminable context conditioning paradigm was developed that demonstrated both conditioned freezing and fear-potentiated startle in a context paired previously with foot shock, relative to a context in which foot shock had never been presented. Post-training lesions of the central nucleus of the amygdala completely blocked both contextual freezing and fear-potentiated startle. Post-training lesions of the dorsal hippocampus attenuated contextual freezing, consistent with previous reports in the literature; however, these same lesions had no effect on fear-potentiated startle, suggesting preserved contextual fear. These results suggest that lesions of the hippocampus disrupt the freezing response but not contextual fear itself.     

Intra-amygdala infusion of the N-methyl-D-aspartate receptor antagonist AP5 blocks acquisition but not expression of fear-potentiated startle to an auditory conditioned stimulus.

The fear-potentiated startle paradigm, in which the amplitude of the startle reflex is enhanced in the presence of a stimulus previously paired with footshock, was used to measure aversive conditioning after intra-amygdala infusion of the competitive N-methyl-{d}-aspartate (NMDA) receptor antagonist {dl}-2-amino-5-phosphonopentanoic acid (AP5). Infusion of 2.5 μg/side AP5 immediately before 5 noise–footshock pairings on each of 2 consecutive days dose-dependently blocked acquisition or consolidation of auditory fear-potentiated startle, consistent with previous results obtained with a visual stimulus. Somatosensory or auditory transmission deficits do not appear to be induced by intra-amygdala AP5, because rats reacted normally to footshocks and showed reliable potentiated startle expression after pretesting AP5 infusion at a dose that blocked acquisition. Together with earlier reports, these data suggest that an NMDA-dependent process localized in or near the amygdala may be necessary for the acquisition of conditioned fear across different sensory modalities. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lesions of the central nucleus of the amygdala block conditioned excitation, but not conditioned inhibition of fear as measured with the fear-potentiated startle effect.

Lesions of the amygdala block the expression of fear-potentiated startle following either moderate or extensive light?+?shock training. The present experiment assessed whether lesions of the amygdala would also block the expression of conditioned inhibition of fear. Rats were given conditioned inhibition training in which a light was paired with shock and a noise and light compound was presented in the absence of shock. Then half of the rats were given bilateral electrolytic lesions of the amygdala and the remaining rats were sham operated. Lesions of the amygdala blocked the expression of fear-potentiated startle to the light. To assess whether conditioned inhibition was disrupted, rats were retrained with light?+?shock pairings with no further conditioned inhibition training. Amygdala lesioned rats reacquired fear-potentiated startle to the light (Kim & Davis, 1993). Importantly, the noise conditioned inhibitor retained its ability to inhibit fear-potentiated startle to the retrained light. These results indicate that areas of the amygdala critical for initial performance of fear-potentiated startle are not critical for the expression of conditioned inhibition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Fear-potentiated startle in two strains of inbred mice.

The fear-potentiated startle paradigm has been used with great success to examine conditioned fear in both rats and humans. The purpose of this study was to examine fear-potentiated startle in inbred mice. One-month-old C57BL/6J (C57) and DBA/2J (DBA) mice were given tone?+?foot shock training trials. The amplitude of the acoustic startle reflex was measured in the presence and absence of the tone both before and after training. Both strains showed fear-potentiated startle after training as evidenced by larger startle amplitudes in the presence of the tone than in its absence. However, the magnitude of fear-potentiated startle was greater in DBA mice than in C57 mice. These results not only demonstrate fear-potentiated startle in mice for the first time but also suggest that fear-potentiated startle can be influenced by characteristics of the mouse strain. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Normal conditioning inhibition and extinction of freezing and fear-potentiated startle following electrolytic lesions of medial prefrontal cortex in rats.

The authors investigated the role of medial prefrontal cortex (mPFC) in the inhibition of conditioned fear in rats using both Pavlovian extinction and conditioned inhibition paradigms. In Experiment 1, lesions of ventral mPFC did not interfere with conditioned inhibition of the fear-potentiated startle response. In Experiment 2, lesions made after acquisition of fear conditioning did not retard extinction of fear to a visual conditioned stimulus (CS) and did not impair "reinstatement" of fear after unsignaled presentations of the unconditioned stimulus. In Experiment 3, lesions made before fear conditioning did not retard extinction of fear-potentiated startle or freezing to an auditory CS. In both Experiments 2 and 3, extinction of fear to contextual cues was also unaffected by the lesions. These results indicate that ventral mPFC is not essential for the inhibition of fear under a variety of circumstances. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hyperexcitability: Exaggerated fear-potentiated startle produced by partial amygdala kindling.

The present study asked whether partial amygdala kindling would affect the expression of conditioned fear-potentiated startle. Rats were conditioned to be fearful of a light. They were then stimulated bilaterally in the amygdala or hippocampus on 2 consecutive days (partial kindling). Rats were tested 24 hr later for fear-potentiated startle. Amygdala-kindled rats had exaggerated fear-potentiated startle compared to sham-kindled rats. Hippocampus-kindled rats also displayed fear-potentiated startle. but no greater than that of sham-kindled rats. Partial amygdala kindling induced c-fos messenger RNA (mRNA) expression, a marker for neuronal activation, throughout the limbic and neocortices. In contrast, partial hippocampus kindling induced c-fos mRNA in the hippocampus only. The data suggest that kindled-induced hyperexcitability of the amygdala and limbic cortices produced exaggerated conditioned fear-potentiated startle. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Delayed development of fear-potentiated startle in rats.

Examined the developmental emergence of fear-potentiated startle in rats ranging in age from 16 to 75 days. In Exp 1, a pure tone served as the CS and an acoustic startle pulse served as the unconditioned stimulus (UCS) for fear conditioning. Fear-potentiated startle by the tone CS was observed in rats 23 days of age and older but not in rats 16 days of age. In Exp 2, a light served as the CS. Rats 30 days of age and older showed fear-potentiated startle, whereas 23-day-old rats did not. The final experiment demonstrated that another behavioral index of fear, stimulus-elicited freezing, was observed earlier in development than fear-potentiated startle, confirming the effectiveness of the training procedure for conditioning fear. Results suggest that fear-potentiated startle is a relatively late-emerging response system, parallelling the development of conditioned autonomic changes (e.g., heart rate) rather than that of freezing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pretraining Inactivation of the Caudal Pontine Reticular Nucleus Impairs the Acquisition of Conditioned Fear-Potentiated Startle to an Odor, but Not a Light.

Recent data from developing rats suggest that structures downstream from the amygdala are involved in the acquisition of conditioned fear-potentiated startle (FPS). The authors tested this idea in adult rats by temporarily inactivating the structure critical for FPS, the caudal pontine reticular nucleus (PnC), during fear conditioning. When the conditioned stimulus (CS) was an odor, rats displayed freezing, but not FPS, at test. This effect was not due to a decrease in footshock sensitivity. Further, no savings were evident on retraining. When the CS was a light, inactivation of the PnC had no effect on the acquisition of FPS. Thus, the PnC may be crucial for the acquisition of conditioned FPS to an odor, but not a light. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lesions of the bed nucleus of the stria terminalis block sensitization of the acoustic startle reflex produced by repeated stress, but not fear-potentiated startle

1. The effects of lesions of the bed nucleus of the stria terminalis (BST) on the acquisition of conditioned fear were examined. In Experiment 1, BST lesions did not block acquisition of fear-potentiated startle to an explicit visual conditioned stimulus (CS) over 20 days of training. However, BST lesions blocked a gradual elevation in baseline startle also seen over the course of training. 2. The gradual increase in baseline startle was replicated in Experiment 2 without the presence of an explicit CS, using unoperated subjects. Experiment 2 showed that the elevation was due to repetitive exposure to shock, because unshocked control subjects did not show any elevation over sessions. 3. In Experiment 3, lesions of the BST did not disrupt rapid sensitization of the startle reflex by footshock, showing that different neural substrates underlie sensitization of startle by acute and chronic exposure to footshock. 4. These data indicate that the BST, despite its anatomical continuity with the amygdala, is not critically involved in the acquisition of conditioned fear to an explicit CS. Nevertheless, the BST is involved in mediating a stress-induced elevation in the startle reflex. This suggests that the BST and the CeA, which constitute part of the "extended amygdala" have complementary roles in responses to stress.     

Lesions of the perirhinal cortex interfere with conditioned excitation but not with conditioned inhibition of fear.

Posttraining lesions of the perirhinal cortex (Prh) have been shown to interfere with the expression of fear. This study assessed whether Prh lesions would also disrupt the inhibition of fear as measured with conditioned inhibition of fear-potentiated startle. Following light + shock, noise→ light-no shock conditioned-inhibition training, rats were given Prh lesions. The lesions interfered with the expression of fear-potentiated startle to the light. To assess whether conditioned inhibition was affected, the rats were given light + retraining without additional noise→ light - training. The noise-conditioned inhibitor retained its ability to inhibit fear-potentiated startle to the retrained light. These results suggest that the areas of the Prh that are essential for the initial expression of conditioned fear are not important for the expression of conditioned inhibition of fear. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contingency awareness and fear inhibition in a human fear-potentiated startle paradigm.

Fear-potentiated startle is defined as an increase in the magnitude of the startle reflex in the presence of a stimulus that was previously paired with an aversive event. It has been proposed that a subject's awareness of the contingencies in the experiment may affect fear-potentiated startle. The authors adapted a conditional discrimination procedure (AX+/BX-), previously validated in animals, to a human fear-potentiated startle paradigm in 50 healthy volunteers. This paradigm allows for an assessment of fear-potentiated startle during threat conditions as well as inhibition of fear-potentiated startle during safety conditions. A response keypad was used to assess contingency awareness on a trial-by-trial basis. Both aware and unaware subjects showed fear-potentiated startle. However, awareness was related to stimulus discrimination and fear inhibition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Potentiation of the acoustic startle response by a conditioned stimulus paired with acoustic startle stimulus in rats.

The hypothesis that the standard acoustic startle habituation paradigm contains the elements of Pavlovian fear conditioning was tested. In a potentiated startle response paradigm, a startle stimulus and a light conditioned stimulus (CS) were paired. A startle stimulus then was tested alone or following the CS. Freezing behavior was measured to index conditioned fear. The startle response was potentiated on CS trials, and rats froze more in CS than in non-CS periods. In Experiment 1, response to a previously habituated, weak startle stimulus was potentiated. In Experiment 2, response to the same stimulus used as the unconditioned stimulus (US) in training was potentiated. This CS-potentiated response retarded the course of response decrements over training sessions as compared with an explicitly unpaired control group. Conditioned fear is a standard feature of this habituation paradigm, serves to potentiate the startle response, and provides an associative dimension lacking in the habituation process per se. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Neural systems involved in fear and anxiety measured with fear-potentiated startle.

A good deal is now known about the neural circuitry involved in how conditioned fear can augment a simple reflex (fear-potentiated startle). This involves visual or auditory as well as shock pathways that project via the thalamus and perirhinal or insular cortex to the basolateral amygdala (BLA). The BLA projects to the central (CeA) and medial (MeA) nuclei of the amygdala, which project indirectly to a particular part of the acoustic startle pathway in the brainstem. N-methyl-D-aspartate (NMDA) receptors, as well as various intracellular cascades in the amygdala, are critical for fear learning, which is then mediated by glutamate acting in the CeA. Less predictable stimuli, such as a long-duration bright light or a fearful context, activate the BLA, which projects to the bed nucleus of the stria terminalis (BNST), which projects to the startle pathway much as the CeA does. The anxiogenic peptide corticotropin-releasing hormone increases startle by acting directly in the BNST. CeA-mediated behaviors may represent stimulus-specific fear, whereas BNST-mediated behaviors are more akin to anxiety. NMDA receptors are also involved in extinction of conditioned fear, and both extinction in rats and exposure-based psychotherapy in humans are facilitated by an NMDA-partial agonist called D-cycloserine. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pharmacological and anatomical analysis of fear conditioning using the fear-potentiated startle paradigm.

Pharmacological and anatomical analyses of fear conditioning using the fear-potentiated startle paradigm are reviewed. This test measures conditioned fear by an increase in the amplitude of a simple reflex (the acoustic startle reflex) in the presence of a cue previously paired with a shock. This paradigm offers a number of advantages as an alternative to most animal tests of fear or anxiety because it involves no operant and is reflected by an enhancement rather than a suppression of ongoing behavior. Fear-potentiated startle is selectively decreased by drugs such as diazepam, morphine, and buspirone that reduce fear or anxiety clinically. By combining behavioral, anatomical, physiological, and pharmacological approaches, it should soon be possible to determine each neural pathway that is required for a stimulus signaling fear to alter startle behavior. Once the exact structures are delineated, it should be possible to determine the neurotransmitters that are released during a state of fear and how this chemical information is relayed along these pathways so as to affect behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Involvement of NMDA receptors within the amygdala in short- versus long-term memory for fear conditioning as assessed with fear-potentiated startle.

Pretraining intra-amygdala infusions of the NMDA receptor antagonist, D,L-AP5, block fear-potentiated startle in rats tested 24+ hr after training. This may reflect a failure of either acquisition or retention. To evaluate these alternatives, rats were tested for fear-potentiated startle during fear conditioning (30 light-shock pairings [0.6 mA shock]), as well as 1–30 min and 48 hr after fear conditioning. Amygdala lesions abolishes fear-potentiated startle at all train-test intervals. Intra-amygdala AP5 infusions (25 nmol/side) abolished fear-potentiated startle during the long-term test and had partial effects at shorter train-test intervals. When the level of fear-potentiated startle during the short-term test was lowered to that of the 48-hr test (i.e., by training rats with a lower, 0.3 mA footshock), AP5 abolished fear-potentiated startle at each timepoint. Thus, amygdala NMDA receptors appear to participate in the initial acquisition of fear memories. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

"Disruption of contextual freezing, but not contextual blocking of fear-potentiated startle, after lesions of the dorsal hippocampus": Correction to McNish et al (2000).

Reports an error in "Disruption of contextual freezing, but not contextual blocking of fear-potentiated startle, after lesions of the dorsal hippocampus" by Kenneth A. McNish, Jonathan C. Gewirtz and Michael Davis (Behavioral Neuroscience, 2000[Feb], Vol 114[1], 64-76). The captions for Figure 4 (p. 70) and Figure 5 (p. 72) were printed incorrectly. The caption used for Figure 4 should appear under Figure 5, and the caption used for Figure 5 should appear under Figure 4. (The following abstract of the original article appeared in record 2000-13470-005.) The role of the dorsal hippocampus in contextual fear conditioning was investigated with a contextual blocking paradigm. In Experiment 1, rats were given pairings of a light conditioned stimulus (CS) and footshock after preexposure either to footshock or to the context alone. The group preexposed to footshock showed poorer fear conditioning to the light CS, as measured by the fear-potentiated startle reflex. In Experiment 2, a group preexposed to footshock in the same context showed poorer fear conditioning to the light CS than did a group preexposed to footshock in a different context, indicating contextual blocking of fear-potentiated startle. In Experiment 3, lesions of the dorsal hippocampus had no effect on contextual blocking, even though contextual freezing was disrupted. The sparing of contextual blocking indicated that contextual memory was intact following hippocampal lesions, despite the disruption of contextual freezing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Fear-potentiated startle and electrically evoked startle mediated by synapses in rostrolateral midbrain.

Startle amplitudes are increased when acoustic startle responses are elicited in the presence of a stimulus that has previously been paired with shock. This "fear-potentiated" startle response appears to be mediated via the caudal ventral amygdalofugal pathway to the brainstem. Electrical stimulation of this pathway evokes unconditioned startlelike responses. Collision tests have shown that a monosynaptic connection from amygdala to midbrain mediates these responses. Collision tests here localize these synapses mediating electrically evoked startlelike responses to the rostrolateral midbrain in awake rats. To test whether rostrolateral midbrain synapses also mediate fear-potentiated startle, we lesioned cells in these sites with ibotenic acid. These lesions completely blocked fear potentiation of acoustic startle. These same lesions did not block potentiation of startle by d-amphetamine (6 mg/kg). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Amygdala central nucleus lesions attenuate acoustic startle stimulus-evoked heart rate changes in rats.

Amygdala central nucleus (CNA) lesions were used to test the hypothesis that stimulus-evoked heart rate changes can reflect the development of fear during acoustic startle testing A 120-dB white noise startle stimulus produced freezing as well as phasic heart rate accelerations and decelerations, and an abrupt decrease in tonic heart rate, in sham-operated rats. These responses were all significantly reduced in CNA-lesioned rats. In contrast, an 87-dB stimulus elicited only significant phasic decelerations that were similarly attenuated by the CNA lesions. In a follow-up experiment. the CNA lesions also attenuated phasic cardiac decelerations evoked by a conditioned stimulus-like, 85-dB pure tone. The results support the contention (B. J. Young & R. N. Leaton, see record 1995-12737-001) that heart rate changes can reflect fear conditioned during acoustic startle testing and, in addition, suggest that the amygdala mediates responses to nonsignal acoustic stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of the hippocampus, the bed nucleus of the stria terminalis, and the amygdala in the excitatory effect of corticotropin-releasing hormone on the acoustic startle reflex

Previously, we demonstrated that transection of the fimbria/fornix blocked the excitatory effect of corticotropin-releasing hormone (CRH) on startle (CRH-enhanced startle), suggesting that the hippocampus and its efferent target areas that communicate via the fimbria may be critically involved in CRH-enhanced startle. The bed nucleus of the stria terminalis (BNST) receives direct projections from the ventral hippocampus via the fimbria/fornix. Therefore, the role of the ventral hippocampus, the BNST, and the amygdala in CRH-enhanced startle was investigated. NMDA lesions of the BNST completely blocked CRH-enhanced startle, whereas chemical lesions of the ventral hippocampus and the amygdala failed to block CRH-enhanced startle. However, the same amygdala-lesioned animals showed a complete blockade of fear-potentiated startle, a conditioned fear response sensitive to manipulations of the amygdala. In contrast, BNST-lesioned rats had normal fear-potentiated startle. This indicates a double dissociation between the BNST and the amygdala in two different paradigms that enhance startle amplitude. Microinfusions of CRH into the BNST, but not into the ventral hippocampus, mimicked intracerebroventricular CRH effects. Furthermore, infusion of a CRH antagonist into the BNST blocked CRH-enhanced startle in a dose-dependent manner. Control studies showed that this blockade did not result from either leakage of the antagonist into the ventricular system or a local anesthetic effect caused by infusion of the antagonist into the BNST. The present studies strongly suggest that CRH in the CSF can activate the BNST, which could lead to activation of brainstem and hypothalamic BNST target areas involved in anxiety and stress responses.